linux-kernel - Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

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Message-ID: <20160926111511.1d963075@grimm.local.home>
Date:   Mon, 26 Sep 2016 11:15:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top
 waiter

On Mon, 26 Sep 2016 14:32:14 +0200
Peter Zijlstra <peterz@...radead.org> wrote:
> 
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1374,9 +1374,8 @@ static int wake_futex_pi(u32 __user *uad
>  	 * scheduled away before the wake up can take place.
>  	 */
>  	spin_unlock(&hb->lock);
> -	wake_up_q(&wake_q);
> -	if (deboost)
> -		rt_mutex_adjust_prio(current);
> +
> +	rt_mutex_postunlock(&wake_q, deboost);

Hmm...

>  
>  	return 0;
>  }
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1325,6 +1308,16 @@ static bool __sched rt_mutex_slowunlock(
>  	 */
>  	mark_wakeup_next_waiter(wake_q, lock);
>  
> +	/*
> +	 * We should deboost before waking the top waiter task such that
> +	 * we don't run two tasks with the 'same' priority. This however
> +	 * can lead to prio-inversion if we would get preempted after
> +	 * the deboost but before waking our high-prio task, hence the
> +	 * preempt_disable before unlock. Pairs with preempt_enable() in
> +	 * rt_mutex_postunlock();

There's a preempt_enable() in rt_mutex_postunlock()? Does
wake_futex_pi() know that?

-- Steve


> +	 */
> +	preempt_disable();
> +
>  	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
>  
>  	/* check PI boosting */
> @@ -1390,14 +1383,23 @@ rt_mutex_fastunlock(struct rt_mutex *loc
>  	} else {
>  		bool deboost = slowfn(lock, &wake_q);
>  
> -		wake_up_q(&wake_q);
> -
> -		/* Undo pi boosting if necessary: */
> -		if (deboost)
> -			rt_mutex_adjust_prio(current);
> +		rt_mutex_postunlock(&wake_q, deboost);
>  	}
>  }
>  
> +
> +/*
> + * Undo pi boosting (if necessary) and wake top waiter.
> + */
> +void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost)
> +{
> +	wake_up_q(wake_q);
> +
> +	/* Pairs with preempt_disable() in rt_mutex_slowunlock() */
> +	if (deboost)
> +		preempt_enable();
> +}
> +
>  /**
>   * rt_mutex_lock - lock a rt_mutex
>   *
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -111,6 +111,7 @@ extern int rt_mutex_finish_proxy_lock(st
>  extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to);
>  extern bool rt_mutex_futex_unlock(struct rt_mutex *lock,
>  				  struct wake_q_head *wqh);
> +extern void rt_mutex_postunlock(struct wake_q_head *wake_q, bool deboost);
>  extern void rt_mutex_adjust_prio(struct task_struct *task);
>  
>  #ifdef CONFIG_DEBUG_RT_MUTEXES
>